Field of the Invention
The invention relates to systems for communication between surface and downhole equipment in a well, and more particularly to systems and methods for reliably communicating data between surface and downhole equipment over a three-phase power cable, where two data bits and an associated error correction bit are concurrently transmitted on the different conductors for the three different phases.
Related Art
Artificial lift systems such as pumps may be used to produce fluids (e.g., oil) from wells. Electric submersible pumps “ESP's” are commonly used for this purpose. The ESP may be coupled to the end of tubing that can then be used to lower the ESP into the well. The ESP is positioned so that it is located in a portion of the well bore where fluid from the surrounding geological formation flows into the well.
Gauges (sensor packages) can be attached at the bottom of the ESP system to allow various parameters (e.g., motor temperature, fluid temperature, fluid pressure, etc.) to be monitored and communicated to equipment at the surface of the well. In order to make use of the information obtained through the gauges, it is necessary to be able to communicate this information from the gauges downhole in the well to the surface of the well, where display, data collection and control systems for the ESP system are located.
This information can be communicated, for example, via dedicated communication lines or via “comms on” transmissions over the power cable. In comms on power systems, data transmissions are impressed upon the power cable so that a dedicated data line is not necessary. There are a variety of techniques for transmitting data over power cables that range from simple low frequency current loop techniques to more complex methodologies, such as orthogonal frequency division multiplexing (OFDM).
Over time, the effects of fluids and the well environment cause the performance of the communication system in a well to degrade. Bit errors can occur singly, repetitively, or in bursts. The more the system degrades, the more likely it is that repetitive bit errors will occur. This may increase the likelihood of random and repetitive noise and, at some point, effective communication of data between the downhole equipment (e.g., ESP) and the surface equipment can be lost. This may occur well before the operation of the ESP is affected. When the system is operated without the benefit of information obtained through monitoring, the system may suffer damage and downtime that could have been avoidable if the information were available.
Conventional comms on power protocols cannot handle a single bit error in an entire message. Currently, the total number of bits in a comms on power message can exceed 100 bits, and if just one error occurs, the entire message is discarded as being corrupted. It would therefore be desirable to provide means to enable a comms on power communication system to continue function effectively despite degradation that may result in one or more bit errors in received comms on power messages.